One of the main problems with the use of a conventional reactor to convert liquid and hydrocarbon fuels to hydrogen for use in fuel cells or for industrial applications is that the hydrogen is produced as an impure mixture. A purifier, either using membranes or pressure swing absorption often is used in-line after the reactor, but a better solution for many applications is a membrane reactor, a device that combines a hydrogen generating reactor and a hydrogen extracting membrane. Membrane reactors combine in one vessel a reaction, that often is catalyzed with a membrane that extracts a product or introduces a reactant. Such reactors have advantages over conventional reactors especially for applications like converting liquid hydrocarbon fuels to hydrogen for use in fuel cells or for chemical applications. R. E. Buxbaum, Journal of Separation Science, 1999. With a suitable membrane, a membrane reactor produces ultra-pure hydrogen and allows the endothermic forming reaction to go forward at higher pressures and lower temperatures than would be feasible otherwise. Membrane reactors of this type are illustratively described in U.S. Pat. Nos. 4,810,485; 5,888,273; 6,183,543 and 5,931,987.
In membrane reactors such as those identified above, an appropriate feedstock material such as methane-water, methanol-water or ammonia is heated to boiling outside of the reactor and reacted in the presence of a reaction catalyst. Hydrogen as well as undesirable gases are produced, but only the hydrogen is extracted through the membranes.
In these prior art reactors the catalyst is distributed within the reactor housing such that catalyst is in contact with a membrane making horizontal orientation of the reactor apparatus difficult because reaction catalyst displacement causes lower efficiency gas collection.
The hydrogen output of the reactor is determined in large part by heat transfer to the reaction catalyst and to a much lesser extent by permeation in the membrane or specific activity in the reaction catalyst. Heat transfer is increased temporarily by using higher temperature heating gases, for example, and reaction rates rise as expected, but this solution often harms the reaction catalyst and can reduce the overall thermal efficiency as well. Thus, there exists a need for a membrane reactor that achieves better thermal integration between a heat source and a reaction catalyst.
Another problem typically encountered with these reactions is in finding an efficient method to compress the feed to the reactor and exhaust a bleed stream of desired gas from the fuel cell. This is important especially with small reactors that feed hydrogen to small mobile fuel cells. The pump that compresses the feed uses a large amount of electric energy, thereby reducing the efficiency of the overall system. Also, most fuel cells are constrained to run with hydrogen above atmospheric pressure as there is currently no convenient way to exhaust impurities that enter the hydrogen by diffusion through the fuel cell membrane. Thus, there exists a need for a reactor having efficient mechanisms to compress the feed, exhaust the fuel cell, or both.